Platelets interact with the coagulation and fibrinolysis systems in the maintenance of hemostasis and in the pathogenesis of thrombosis and thromboembolism. Platelets rapidly adhere to damaged vascular tissue, and release a variety of prothrombotic, chemotactic, and mitogenic factors, aimed at prompting hemostasis and wound healing. Platelets also play an important role in arterial thrombosis, a common cause of death and disability in patients with cardiovascular disease. Platelet inhibitors have been successfully used for secondary prevention of arterial thrombosis in patients with coronary, cerebral, and peripheral vascular disease.
Platelets adhere to exposed subendothelium after vessel wall injury by binding to von Willebrand factor (vWf) and collagen. This induces platelets to change shape from a disc shape to a round form with pseudopodia, which enforces platelet adhesion and aggregation. The final common pathway for platelet aggregation is the activation of the fibrinogen receptor (GPIIb-IIIa). As a result, dimeric fibrinogen molecules present in plasma can bind and link platelets together to form aggregates.
Activated platelets secrete their granule contents, many of which act directly on blood cells, including platelets themselves, and endothelium. Platelets contain several kinds of secretory granules. The dense-granules contain adenosine diphosphate (“ADP”), adenosine triphosphate (“ATP”) and serotonin. The α-granules contain several platelet-specific proteins (platelet factor 4 and β-thromboglobulin), growth factors (PDGF, TGF-β, EGF and ECGF) and coagulation factors (fibrinogen, Factor V and vWf). Platelets also secrete biologically active arachidonic acid products. Well known is TxA2 which is inhibited by aspirin through irreversible inactivation of the cyclooxygenase producing TxA2.
Many stimuli, such as thrombin, collagen, ADP and thromboxane A2 (TxA2), activate platelets by binding to their cell surface receptors. Most of these receptors are G-protein-coupled receptors. Activation of G-proteins has been shown to be an essential event in platelet activation. For example, platelets from Gq-/- mice do not aggregate in response to thrombin, collagen, ADP or TxA2 (Offermans, S. et al., Nature (1998), Vol. 389, No. 11, pp. 183–185). Many down-stream signaling events have been elucidated, including activation of phospholipase-C (PLC) and protein kinase C, increase in intracellular calcium concentration, decrease in cAMP level and tyrosine phosphorylation.
ADP plays a pivotal role in platelet activation. ADP not only causes primary aggregation of platelets but is also responsible for the secondary aggregation following activation by other agonists such as thrombin and collagen. Contained at very high concentrations in the platelet dense-granules, ADP is released when platelets are activated to reinforce platelet aggregation.
ADP-induced platelet activation plays an important role in maintaining normal hemostasis. Several congenital bleeding disorders have been linked to the decreased number of platelet ADP receptors and deficiency of ADP-induced platelet aggregation. Patients having “storage pool disease”, which is due to defects in the storage of nucleotides and/or their secretion from the platelet dense-granules, have impaired platelet aggregation in response to collagen and other stimuli due to the absence of the amplification effects by ADP.
ADP-induced platelet activation also plays a key role in the initiation and propagation of thrombosis. Administration of ADP has been shown to induce thrombus formation in rat and mice mesenteric venules. In contrast, ADP-removing enzymes have been shown to dramatically reduce platelet deposition on collagen and to inhibit laser-induced thrombosis in rat mesenteric arterioles and venules, supporting the theory that ADP plays a role in mediating platelet recruitment in thrombus formation. Several ADP-induced early signaling events in platelets have been described. These include a transient rise in free cytoplasmic calcium, an inhibition of adenylate cyclase through activation of GI2, an increase in cytosolic pH by activating the Na+/H+-exchange, and exposure of the platelet binding sites for fibrinogen independent of protein kinase C. While these signaling events collectively contribute to platelet aggregation, the specific role of each remains the subject of on-going investigations.
The current model of ADP-induced platelet activation involves two G-protein coupled purinergic receptors, one of which is coupled to the activation of the phospholipase-C pathway (P2Y1) and the other is coupled to the inhibition of adenylate cyclase (P2YAC). P2YAC is the best target for a platelet ADP receptor antagonist for several reasons. First, P2YAC is predominately platelet specific. Secondly, it is required for ADP-induced aggregation. Thirdly, it plays an important role in sustaining thrombin or collagen-induced aggregation. Finally, it is the molecular target for anti-aggregatory drugs such as Clopidogrel and Ticlopidine. Both of these drugs have been shown to be efficacious in various thrombosis models. However, Clopidogrel has been shown to be an irreversible inhibitor of platelet aggregation with a slow onset of action. Similarly, the ATP analogues, AR-C67085 and AR-C69931MX, which are potent antagonists for ADP-induced platelet aggregation, have also been shown to be effective in thrombosis models and are currently under clinical investigation. All these findings indicate that ADP is a critical mediator of arterial thrombus formation and hence an excellent target for antithrombotic intervention.
When the properties of current oral platelet inhibitors, such as aspirin, Clopidogrel and Ticlopidine are compared, it becomes clear that, while relatively safe, current oral platelet inhibitors are only modestly effective in preventing thrombotic complications in patients with underlying vascular disease. It is clear that there is a need in this field for a potent, selective, reversible, orally active platelet ADP receptor (P2YAC) inhibitor.